[Haskell-cafe] tips on using monads

Claus Reinke claus.reinke at talk21.com
Mon May 18 12:58:50 EDT 2009

>>> I've got one of those algorithms which "threatens to march off the right edge" (in the words of 
>>> Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. 
>>> So I presume I need StateT or MaybeT. However, I'm still (sdlowly) learning about monads from 
>>> first principles. I thought I might present my code and get some pointers... maybe someone could 
>>> actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in 
>>> action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything 
>>> will help.
>> Perhaps this is useful:
>> http://www.haskell.org/haskellwiki/Equational_reasoning_examples#Coding_style:_indentation_creep_with_nested_Maybe
> I can't quite tell--is that example in the IO monad? Part of my difficulty is that I'm inside IO. 
> I know how to do this with Maybe, except that I have to combine Maybe and IO (use MaybeT?)

It was in the GHC.Conc.STM monad, so yes, it used a MaybeT
and Control.Monad.Trans.MonadTrans's lift (btw, the MonadTrans
docs only point to [1], but [2] might also be of interest, if rather
more compact/terse).


[1] http://web.cecs.pdx.edu/~mpj/pubs/springschool.html
[2] http://web.cecs.pdx.edu/~mpj/pubs/modinterp.html

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